Compositions and methods for chemical-mechanical polishing (CMP) of the surface of a substrate are well known in the art. Polishing compositions (also known as polishing slurries, CMP slurries, and CMP compositions) for CMP of metal-containing surfaces of semiconductor substrates (e.g., integrated circuits) typically contain an abrasive, various additive compounds, and the like.
In general, CMP involves the concurrent chemical and mechanical polishing of an overlying first layer to expose a portion of the surface of a non-planar second layer on which the first layer is formed. For example, U.S. Pat. No. 4,789,648 discloses a CMP process using a polishing pad and a polishing composition to remove a first layer at a faster rate than a second layer until the surface of the overlying first layer of material becomes coplanar with the upper surface of the covered second layer. A more detailed explanation of chemical-mechanical polishing can be found in U.S. Pat. Nos. 4,671,851, 4,910,155 and 4,944,836.
Manufacturers of integrated circuits strive to improve the current density of semiconductor devices. It has become necessary to use conductive materials having low resistivity for conductors in feature definitions formed in materials having low dielectric constants as insulating layers to reduce the capacitive coupling between adjacent interconnects. A conductive material fitting these requirements is copper and alloys thereof.
One difficulty in using copper (Cu) in semiconductor devices is that copper diffuses into surrounding insulator material. To reduce the copper diffusion into the insulator material, and to aid in the adhesion of copper, a barrier layer is deposited in feature definitions prior to copper deposition. Barrier materials include, for example, tantalum (Ta), tantalum nitride (TaN), titanium (Ti), and titanium nitride (TiN). Following copper deposition, the excess copper and barrier layer is removed using CMP.
Current CMP processes and commercially available slurries for barrier layer removal are limited as to the useful chemical composition of the slurry because of the relatively inert nature of Ta. Consequently, polishing relies predominantly on strong mechanical abrasion. Stated somewhat differently, the currently available CMP processes and compositions with high solids concentrations for workpieces with Ta-containing barrier layers have very poor selectivity among the barrier layer, metal layer (Cu-based), and the interlayer dielectric (ILD) layer (silicon oxide-based), resulting in excessive concurrent removal of the metal and ILD layers.
Another approach to polishing multiple metals in a substrate is to use a large amount of oxidizer sufficient to oxidize all the metals to be polished on the substrate. The removal rates of the metals are then controlled by the use of additives which passivate a metal surface or complex a metal ion. This approach requires optimizing the passivation film chemistry to slow down the oxidation of one metal and allow the removal of a second metal on the substrate.
Many of the known CMP compositions are suitable for limited purposes but also suffer from unacceptable polishing performance. Thus, there remains an ongoing need for new CMP compositions that exhibit useful removal rates for semiconductor materials such as tantalum when present with a second metal.